1. Field of the Invention
The present invention relates to a quasifixed variable resistor for which a fine adjustment can be effected by use of a screwdriver of an automatic adjuster from a rear surface of the resistor, for example, during or after an assemblage of an electric circuit.
2. Description of the Prior Art
Although the process for mounting circuit elements including a quasifixed variable resistor on a printed circuit board has already been automated, the adjustment of the resistance value has been recently implemented by use of an automated system. In this method, a screwdriver included in the automated system for the adjustment is linked to an adjusting section of a variable resistor and the adjustment is achieved by rotating the screwdriver while checking the resistance value or the circuit constant.
Moreover, conventionally, the automatic adjustment is accomplished through an adjusting section disposed in the front surface (from the side of the front surface, the variable resistor is visible when the variable resistor is mounted on the printed circuit board) of the variable resistor. It has been recently required, however, to use an adjusting section disposed in a rear surface (from the side of the rear surface, the variable resistor is invisible when the variable resistor is mounted on the printed circuit board because it is interrupted by the printed circuit board) of the variable resistor, namely, a screwdriver is inserted through a hole formed in the printed circuit board and is linked to the adjusting section; thereafter, the automatic adjustment is effected.
FIGS. 4 and 5 are cross-sectional views of a conventinal quasifixed variable resistor for which the adjustment can be achieved through the rear surface.
The first conventional example of FIG. 4 includes a frame 21 made of, for example, a mold, a resistor substrate 22, a terminal 23 connected to both ends of a resistor, an intermediate terminal 24, and a knob 25 having an adjusting groove 26 on a surface thereof. The configuration further comprises a slider 27 fixed to the knob 25 and an adjusting rod 28 rotatably retained in the frame 21, the rod 28 having an end fixed to the slider 27 and the knob 25. Reference numeral 29 indicates an adjusting groove disposed in another end of the rod 28. A screwdriver (not shown) of the automatic adjuster is inserted into the groove 29 through the rear surface of the printed circuit board 30 shown by use of a double-dot-and-dash line and is rotated to achieve the adjustment.
In addition, as shown in the second conventional example of FIG. 5, there has been a type in which an end of the screwdriver is inserted into a hollow cylinder section 32 of a stopper plate 31 and then the screwdriver is further inserted into a hole 34 disposed in a knob 33; thereafter, the adjustment is achieved.
According to the first conventional example since described above, since the outer shape of the driving device of the automatic adjuster is usually rounded, the engagement thereof with the groove 29 having a small depth is difficult and the adjustment is impossible; consequently, there arises a disadvantage that the thickness of the adjusting section is increased if a groove 29 with a larger depth is used.
In the second conventional example, since the cylinder 32 of the stopper plate 31 has a small diameter and the hole 34 through which the screwdriver is inserted is also quite small, it may be possible that the hole 34 is damaged, which may also disable the adjustment. Furthermore, the screwdriver must be inserted into a deeper location and a great amount of travel must be made by the screwdriver of the adjuster; consequently, the structure of the adjuster becomes complicated and a long ajusting time is necessary for effecting a difficult adjusting operation.
In addition, for the type of the adjustment using a standard or slotted screwdriver as in the examples above, the period of time required when the tip end of the screwdriver is engaged with the adjusting section is twice that required for the type of the adjustment using a Phillips screwdriver; consequently, it may be considered to modify the adjusting section of these examples to a type of the adjustment using a Phillips-head screwdriver. In this case, however, for the type of the first conventional example, the thickness of the adjusting section is increased, whereas for the type of the second conventional example, it is difficult to modify the adjusting section itself to be of the type of the adjustment using a Phillips screwdriver.